Nature's first green is gold,
Her hardest hue to hold.
Her early leaf's a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.
Maple trees-- with their early sprays of brilliant yellow-green blossoms-- may have been the picture Frost had in mind when he wrote these four couplets. The theme is a classic one: the blending of grief and joy we feel when delightful things pass away all too quickly. It appears in some of the best poems we have. A few that spring to mind are George Herbert's "Virtue", John Keats's "To Autumn", and Gerard Manley Hopkins's "Spring and Fall".
A good poem doesn't always give us a new thought, but it does always find its own way of embodying a thought-- of placing us right there, in a particular feeling, so the the feeling lives, really happens, within the words as we read them. (I think this is what Marianne Moore meant by her "real toads".)
The way this poem does this-- gives actuality and life to a feeling-- has to do with the couplets. Look at how they work: they start off neatly, with lines paired in both rhyme and meaning. So we have a rhyming pair on the topic of the green which is really gold. Then a new rhyming pair on the topic of a leaf which is really a flower. But in the next couplet things start changing. The first line-- "Then leaf subsides to leaf"-- is linked to the preceding line: it builds on the meaning of "But only so an hour." The second line-- "So Eden sank to grief"-- is not linked to its rhyming counterpart. Instead it introduces something new: an analogy to an ancient story; it is linked in meaning to the line that follows, a parallel "so" analogy. And the last line is left standing alone; there is nothing left for it to do but carry us back to the title with its inevitable truth.
Just in reading these lines we've experienced a loss, a sense of something slipping away from us. At first the rhyming lines are coupled squarely-- we have a grasp on them-- but then they begin to escape our hold and go their own way. As they do, though, they take on a different beauty, something less static, more vital. What we feel in the motion of the lines-- the blending of enjoyment and loss-- is just what the words are telling us about.
My description of this process, of course, is plodding and dry, while the process itself is quick, flowing, subtle. One of John Keats's "axioms" for poetry was that "if poetry comes not as naturally as the leaves to a tree it had better not come at all." "Nothing Gold Can Stay" passes the test.
There is a lot more to be said about this poem. Here's a sampling.